1. Field of the Invention
The present invention relates to a method for causing transition from perovskite phase to pyrochlore phase, a method for producing a lead niobate-based complex oxide with an increased ratio of perovskite phase utilizing said phase transition method, and a novel lead niobate-based complex oxide with an increased ratio of perovskite phase, produced by said method.
2. Related Background Art
Lead niobate-based complex oxides represented by a general formula (3): Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 and called PZN, and those represented by a general formula (4): Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 and called PMN are recently attracting attention as ceramic materials (signifying sintered materails) for producing small actuators capable of submicron displacement and small capacitors. The crystal structure of these oxides can exist in perovskite type or pyrochlore type. Actual PZN or PMN contains both crystal structures in mixed manner, unless it is monocrystalline, and said structures are respectively called perovskite phase and pyrochlore phase.
In both oxides, the perovskite phase shows a high dielectric constant k. In general, the dielectric constant k shows a peak-shaped change as a function of temperature, and the temperature showing highest value of k is called Curie temperature Tc. A material with a high dielectric constant k shows a piezoelectric effect in a temperature region lower than Tc, and an electrostriction effect in a temperature region higher than Tc. PZN, with Tc at 150.degree. C., shows piezoelectric effect at room temperature, while PMN, with Tc at -10.degree. C., shows electrostriction effect at room temperature.
In single crystals, PZN or PMN has been obtained with the ratio of perovskite phase at approximately 100% (namely the ratio of pyrochlore phase approximately 0%).
However, in the sintered material or the raw material powder therefor (generally produced by one-step solid state reaction from metal oxide), PZN has been obtained with the ratio of perovskite phase almost 0%, and PMN has been obtained with said ratio only up to 70-90%.
The ratio of perovskite phase in the powdered or sintered state is important because of the method for producing small actuators or capacitors.
In general, the method for producing small actuators and capacitors consists of mixing powdered PZN or PMN (particle size about 1 .mu.m) with an organic solvent, a resinous binder etc. to obtain slurry, then preparing from said slurry a tape of a thickness for example of 200 .mu.m (so-called green tape) from which the organic solvent is already evaporated, subsequently forming a laminate product by repeating a step of cutting the green tape into a predetermined shape and screen printing electrodes thereon, heating said laminate product at 400.degree.-500.degree. C. to burn off the binder, further sintering the product for example at 1000.degree.-1200.degree. C. and finally attaching external electrodes by heat treatment.
For such method, powdered PZN or PMN is required as the raw material. However the powder obtained by crushing perovskite single crystals cannot be used for such purpose, because the single crystals are extremely expensive.
It was also reported that sintered PZN with a perovskite content of ca. 90% could be directly obtained by a solid-state reaction of powdered mixture of various metal oxides constituting PZN under a high pressure of ca. 25,000 kg/cm.sup.2 (cf. Yogyo Kyokaishi, Vol. 78, No. 2, p. 46-58). PZN powder with an elevated ratio of perovskite phase can be obtained by crushing said sintered PZN. However, this method is associated with a drawback of an extremely high production cost, because it requires an ultra-high pressure generating apparatus such as a diamond anvil, and the processed material has to be hermetically sealed in a container for example of platinum.
For this reason, there has been developed a method of forming solid solution of PZN with PMN which can relatively easily assume the perovskite structure, as described in the Japanese Patent Application Laid-Open Nos. 57-25607 and 57-27974. Said solid solution can be produced by sintering a powdered mixture of various metal oxides constituting PZN and PMN for 1-3 hours at 700.degree.-900.degree. C. under atmospheric pressure.
Said solid solution of PZN and PMN is represented by a general formula: EQU Pb(Mg.sub.1/3 Nb.sub.2/3).times.(Zn.sub.1/3 Nb.sub.2/3).sub.y O.sub.3( 5)
wherein x is an atomic ratio of 0.4-1 and y is an atomic ratio of 0.6-0 with a relation x+y=1, and contains the perovskite phase in a ratio of 0-90% depending on the value of x or y. It is thus rendered possible to obtain a sintered material containing the perovskite phase from the powder of said solid solution, and to prepare devices of resonable performance by the above-explained method.
However, a lowered proportion of the perovskite phase, whether in PZN or PMN alone or in said solid solution, accordingly reduces the dielectric constant of the sintered material, thereby deteriorating the performance of obtained devices.
Thus, there has also been developed a method of forming solid solution of PZN with PbTiO.sub.3 (so-called PT) which assumes perovskite crystal structure, for example as disclosed in "Solid and Powder Metallurgy" Vol. 16, No. 6, p. 253. Said solid solution in monocrystalline state shows maxima of dielectric constant and electromechanical coupling constant and exhibits excellent piezoelectric property in compositions around 0.9PZN-0.1PT. However, in ceramic state, the dielectric constant is lowered and the sintering becomes impossible. This is due to the mixed presence of perovskite and pyrochlore phases in the ceramics of a composition with PZN of 50 mol. % or higher. In the PZN-PT ceramics, a low proportion of the perovskite phase accordingly reduces the dielectric constant of the sintered material, thereby deteriorating the performance of the obtained devices.
Because of the ever increasing requirement for the performance of devices in recent years, it is being longed for to obtain a higher proportion of the perovskite phase in PZN powder, PMN powder, PZN-PMN solid solution and PZN-PT solid solution which are not obtained by crushing the perovskite single crystals but by an ordinary solid phase reaction.